We could cut residential heating and cooling equipment size in America by 30% to 50% if Contractor's would perform honest Manual J calculations and provided full credit for every load reducing element or detail they can when doing the calculation audit.
Additionally, load reduction remedial actions should be provided as options toward further reducing Air Conditioning and heating equipment sizing.

Contractors' should use the equipment manufacturers blower data and the Manual D procedures to find the room cubic feet per minute (CFM) airflow values and then use published performance data to select the appropriate sized Supply-Air outlet & Return-Air ducting & grilles, type and size, for each room. There also should be a low resistance return air path for every room that has a supply outlet, door undercuts are borderline acceptable.

Manual D procedures should be used to size all the duct runs, - and systems should comply with ASHRAE standards; completely seal all runs located in an unconditioned space and insulate these runs to preferably R-8.

Contractor's should Certify the work they have done, i.e., —measured all air flows, balanced the air distribution system and then used certified protocols (superheat & Subcooling) to check and balance the refrigerant charge. After all the installation work has been done, the Operating Performance Standard Data of the operating System should be Certified by the contractor. This should Include the static pressure readings, CFM of the system airflow, air temperature rise across the condensing coils, and the entire performance data. Provide your customers with more than they paid for and you will have more business and solid referrals.
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Because the new high Seer efficiency air conditioners “have a smaller tonnage compressor” for their capacity rating, then do the lower SEER rated units that are being replaced. Therefore, the ultra high SEER units do not achieve as high an the indoor temp-split. The older lower SEER units could achieve a 4 to 6-F lower evaporator coil temp under the same indoor conditions. The colder coil of the lower SEER units did a better job of removing humidity under some indoor conditions.

Instead of doing things that would reduce the heat-gain/heat-loss, I tend to believe that they look at the capacity of the installed unit and then up the tonnage one-half to a full-ton, perhaps because they are concerned about the smaller compressor per tonnage rating handling a high heat-load.

Far too many unknown realities need to be exposed in order to size the new equipment at the proper tonnage. For example, the ductwork system may not be up to "Manual D standards." If the cubic feet for minute airflow were to fall below 325-CFM per-ton, the unit will not deliver anywhere near its rated capacity.

If the original installation was not done properly, the unit will not perform to its full capacity.
The  home's heat-gain/heat-loss should be reduced using all the methods that will produce a reasonable payback. Then a heat load Calc manual J. should be done, after which, a manual at us for equipment sizing should be done.

If you want optimal seer ratings performance is better to slightly undersized the equipment than do oversized it. The reason for this is the slight undersizing will reduce the frequency of the on-off cycling of the condenser, which is a major factor in the (SEER) “Seasonal Energy Efficiency Rating.”

Low energy usage 20” floor fans will keep you comfortable even with a two to three or more temperature differential spread. The longer run cycles are terrific for reducing humidity.

Another way to improve the SEER ratings performance is to install a digital thermostat with adjustable differential temperature set-point, which can be set to reduce the number of on/off cycles per hour, per day and per cooling season. Additionally, a variable speed blower/air handler and a time delay blower relay will also improve the SEER.

There are too high a percentage of systems that have improperly insulated, sized, & leaky ductwork with inadequate airflow, which leads to an improperly charged system, which causes extremely poor performance and very high utility bills.

Measuring Low Airflow

If you can't afford a flow hood, also called a capture hood, then buy an anemometer & do the math.

Get a low cost Testo Tester & ballpark figure actual BTUH & EER - the information on it:
http://www.amazon.com/Testo-605-H2-H.../dp/B000774B6A

Everyone, very low cost anemometer to get airflow FPM Velocities, get it:
http://www.amazon.com/Crosse-Technol.../dp/B0002WZRKE

Also, get a low-cost digital flat-headed pocket Thermometer to use flat on the piping; these test instruments will PAY big returns!
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This should be helpful: (Edited 6/15/10)
Take the Supply Air & Return Air wet bulb temperatures & interpose them on the enthalpy chart linked below.
Duct system CFM X* 4.5 @sea-level, or use X* 4.35 if 1000' above sea-level, X* change in enthalpy = BTUH (Ballpark) Operating Performance.
"U Must Right Click Link & open in New Tab," look-up wet bulb enthalpy figures on chart," & figure enthalpy change.
Wet Bulb Enthalpy Chart

Rules of Thumb for Duct Systems  - Hart&Cooley
DESIGN AND INSTALLATION OF RESIDENTIAL FLEXIBLE DUCTWORK SYSTEMS
http://www.dca.state.ga.us/development/constructioncodes/publications/1ONE.pdf

Look at the ducting, if it is not to code; make hard copies of this code & give it to whoever does the ducting work
Make sure they redo it right!
Never have flex duct interiors commercially cleaned, I just viewed Home Inspection photos showing the interior damaged & insulation plugging the duct.
Home Inspectors warn people because the duct cleaner's tell them it won't damage the ducts. Some HI's look into the boot areas for clues of problems...
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First, before doing anything else check the sizing, and thoroughly seal and properly insulate all the ductwork!

Design Engineering and Installation Objectives should be focused towards achieving the most efficient and effective means toward  a conditioned space that is within the "Human Comfort Zone, and within an affordable investment 'payback' period." 

Summer Comfort Zone
Relative Humidity	Maximum Comfortable Temperature	Minimum Comfortable Temperature
60%	78.5oF	72.5oF
50%	79ºF	73ºF
40%	79.5oF	73.5oF
30%	80oF	74oF

The above comfort zone was found to be acceptable to 90% of test subjects drawn from a range of age groups and genders, with work and life-styles involving varying levels of activity and clothing. An air conditioning system that establishes and maintains indoor conditions within this zone will provide thermal comfort. It will produce a neutral sensation, occupants will feel neither too hot nor too cold. Above chart and findings From: Home Energy Magazine Online September/October 1996) Sizing Air Conditioners: If Bigger Is Not Better, What Is?  by John Proctor and Peggy Albright  Toward Optimal Occupant Comfort 

If you over pay for over capacity equipment, --you will be paying more every month and will not be as comfortable as you would sizing it right to also achieve the appropriate humidity levels!

When a typical HVAC contractor quotes the efficiency of the Air Conditioning equipment's SEER & Btu/hr, and leads you to believe the new equipment will automatically deliver that SEER efficiency & Btu/hr rating, think again. Typically, --installed equipment only operates at 55% to 70% of rated capacity. Oversized equipment is the worst combination there is because the duct system airflow and heatload on the cooling coil are often way off what is required!

Equipment Ratings are only the 'potential efficiency' of that component of the system under perfect conditions." Over half of the system's efficiency depends on correct equipment sizing toward adequate run-time, on the duct system sizing, i.e., on the quality of the complete field-installation!

What you want & need is right sized equipment operating at its optimal ratings within varying conditions, for your optimal comfort and savings.

If all contractor's would do the above, coupled with installing equipment sized according Manual J loads (with no safety factor), along with Manual S selection procedures, comfort would go up, humidity control would improve, and installation and operating costs would be much lower.

Utility demand loads could be cut by at least a third, or even up to a half. Energy loads would be significantly reduced, reducing our nation's energy usage. The return on the time and effort invested on this higher quality level of work would be tremendous - for customers, the community and the nation.

Unfortunately, most HVAC contractors don't use these procedures to size equipment and design duct systems. It's estimated that only 10% of heating and cooling equipment sizing decisions are based on some type of Manual J calculation and that less than 1% of the jobs are based on an aggressive accurate implementation of these recommended design procedures.

Many if not most contractors are designing new and replacement systems that feature oversized equipment, "improperly sized supply outlets" and duct runs that are too small, too leaky and inadequately insulated.

The Manual J gives appropriate answers if you use an “aggressive” set of assumptions. However, most HVAC contractors tend to skew input data to make the calculations match their favorite rules of thumb. Follow the manual J rules and you will get a reasonable margin of safety. However, after skewing the numbers, many contractors throw in an extra half ton or more of A/C to feel safe. No wonder a large percentage of equipment is considerably oversized. Also, the airflow is usually so compromised on the oversized units that it isn't putting out many more btuh than properly sized equipment would be, but it's an energy waster and is costing and arm and leg to operate.

Especially if your system is oversized or there are a lot of low AC load days use an adjustable differential room TH.
TH Differential: Differential is defined as the difference between the cut-in and cut-out points as measured at the thermostat under specified operating conditions. For example, if the thermostat turns the COOLING EQUIPMENT ON AT 78-F & OFF at 76-F that is a 2 degree differential setting; heating equipment on at 70 degrees F and turns the heating equipment off at 74 degrees F, then the differential is 4 degrees F. Some have half degree increment settings over several degrees of differential spread.

For example, in Rockford, IL a 2,400 sq./ft home with 600 sq./ft of window area, it figures to take 4 tons or so to cool it. However, a 2-Ton Unit moving 1,000-CFM of air (or 500-cfm per ton of cooling), even at 95 degrees with a blazing sun heat outside and very high humidity the 2-ton cooling equipmentsystem still cycles! It is very comfortable at around 75-F and 50% Relative Humidity.

There's a 2400 sq./ft home in Lancaster (SW WI)and one I know of in Ohio, cooling the homes to very comfortable levels using 2-ton A/C systems.

Okay, with 8 foot ceilings, 19200 cu ft air volume at 1000-cfm that's 60000 cu ft per hour, or just over 3 air changes per hour, with the added long run times to reduce humidity. Would you rather have the costly oversized 4 Ton Unit?

Perhaps the Manual J could be improved; however, if used with "integrity" it can deliver good results. However, contractors that don't want to size according to an honest manual J calculation simply change some of the inputs to make the procedure deliver answers they feel safe with, and, --they are never challenged.

First, you need a tight, well insulated building with good windows. Then you need to make sure the person performing the load calculations uses accurate information, and doesn't “fudge or skew the numbers." After they have the equipment sizing answer, "they must be certain the ductwork is properly sized, sealed and well insulated." It's not rocket science and it is time all Contractors are held to a new set of codes and ethical standards.

Take the condenser entering air temp and leaving air temp, subtract for the temp-split. As a double varification:You can use the high-side (SCT) Saturated Condensing Temperature minus the outdoors-ambient temperature; the difference gives you the condenser temperature-rise or temperature/split. There is NO excuse for not utilizing this important diagnostic check. Always use an accurate volt meter and amprobe to make sure you are not overloading the compressor's Wattage Service Factor and check the compressor discharge line to see that it is under 225-F.

CONDENSER TEMP-SPLITS - My Brother's Heil 12-SEER Condensing Unit
1½-Ton - Rated at 18,400-BTUH,  Condenser fan CFM 1400 (Total Cond. Watts 2221 X's power Factors 0.85 X's= 1887 X's * 3.413 = 6,443-BTUH Motor Heat additive +18400= Motor Power "Rated Gross Heat Ejection" is 24,843-BTUH / 1400= 17.7-F  = 17.5-F Temp Rise Cond/Split. His condenser only gets a 10 to 12 temp rise split, the evaporator appears to be under heat-loaded or, an unbalanced heatload on the DX coil's circuits.

  The new Goodman 13-SEER 1½ Ton Condenser, 2-Ton Evaporator:
At 675-cfm 450-per/ton cooling | 85-F ODB | 63-IWB | 52% RH | 20-F ID Delta T | 18,600-Btuh
201-psig 100-F = 15-F cond. temp split - larger coil areas | 80-psig suction

The probable cause is "an unbalanced airflow heatload through the evaporator coil. "It's a (Thermo Pride OL 11 oil furnace). Those oil furnaces have a very large round heat exchanger that goes to near the top of the furnace, --due to a low basement ceiling the DX coil sets perhaps illegally close to the heat exchanger causing a few of the coil's circuits to be under heatloaded. Since the liquid refrigerant is not completely evaporated it will cause the outlet line that the TEV sensor bulb is on to be too cold and the TEV will shut-down the flow, which greatly reduces the BTUH capacity of the DX coil and the system. On piston refrigerant control systems, they may flood back liquid which could damage the compressor, unless the system is way under-charged. Thermo Pride could install airflow turning vanes just above the heat exchanger to funnel the air directly into the DX coil, instead of most of the airflow hitting the bottom of the DX's drain panturbulence back-pressure and an imbalanced DX coil circuitry heatload!

Many DX coils are delivering half to a ton less btuh than the rating of the condenser. Either remedy the problems or install a smaller condenser sized to the achievable DX coil's nominal heat transfer limits.

Do your own figuring based on this formula. Get the Motor Power Factors (PF) of the compressor and fan motor from the manufacturers.

CONDENSER TEMP-SPLITS - Comfortmaker® 12-SEER units - used 0.88 Motor Power Factors

2-Ton  24,800  23-F T.-Split  Cond. CFM 1400     WATTS 2659
2.5-T  30,200  20-F Temp-S    Cond. CFM 2000     WATTS 3404
3-Ton  35,600  17-F Temp-S    Cond. CFM 2800     WATTS 4107
3.5 T  42,500  19-F Temp-S    Cond. CFM 2800     WATTS 4554
4-Ton  48,500  18.5-F Temp-S  Cond. CFM 3400     WATTS 4761
5-Ton  59,000  23-F Temp-S    Cond. CFM 3400     WATTS 6969

The new Goodman 13-SEER 1½-Ton Condenser, 2-Ton Evaporator:
At 675-cfm 450-per/ton cooling | 85-F ODB | 63-IWB | 52% RH | 20-F ID Delta T | 18,600-Btuh
201-psig 100-F = 15-F cond. temp split - larger coil areas | 80-psig suction
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http://www.udarrell.com/air_return_latent_condenser_split.jpg IE Browser's
Page 618, Refrigeration & Air-Conditioning (ARI) Second Edition, © 1987
Those lower SEER units had higher condenser splits than 12-SEER and higher units.
Sorry, I defiled the graph, 90-db outdoor, 80-db indoors with 67 wet bulb/50% RH represents the condenser splits shown above. (Always, check voltage and amp draw!)

Look at all the ways you can to reduce the heat and humidity load, caulk, weather-strip, and insulate, to keep the heat and humidity outside.

With a properly sized system and proper evaporator airflow you will have consistent optimal nominal capacity heat absorption and removal, coupled with longer run-time cycles.

I believe that optimal efficiencies, with variable latent/sensible heat loads, could be effectively achieved through the use of computerized control system components.